A video screen grab shows ISIS executioner Mohammed Emwazi, aka 'Jihadi John,' Nov. 13. The U.S. has claimed he has been killed by a drone stroke in Northern Syria.

Kurdish forces stand guard at a post as the sun rises on the frontline with the Islamic State group in Sinjar, Iraq, Nov. 13. Officials with the Iraqi Kurdish militia said they are preparing to push their large-scale military operation into the center of Sinjar, the strategic mountain town in northern Iraq currently in the hands of the Islamic State group.

A Kurdish peshmerga fighter stands guard in fighting against the Islamic State group as the sun rises in Sinjar, Iraq, Nov. 13. Experts say it would require 150,000 U.S. troops, could last decades and cost trillions to defeat Islamic State.

The Islamic State’s barbaric attacks in Paris are forcing an anguished reassessment by world powers that so far have lacked the political will and regional partners to defeat an organization flush with cash, equipment and volunteers.

As leaders gathered in Turkey on Sunday for a Group-of-20 summit, they pledged to redouble efforts to sap the lifeblood of the terrorists by targeting their finances and recruitment. They will consider deeper intelligence sharing, tighter border controls and the creation of Syrian safe havens.

Some also speak of a much more aggressive military option. Experts say it would require 150,000 U.S. troops, could last decades and cost trillions. It is considered highly unlikely at this stage because it would need to be led by Sunni nations that have shown no appetite for the fight and would pose severe challenges regarding Russia and Syria. Nor would such an invasion address the underlying forces that have shaped Islamic State. “At the heart of it is the failed and broken state system in the Arab world that has given IS space in Iraq, Syria, Egypt and Libya,” said Bruce Riedel, a CIA veteran at the Brookings Institution. “That problem is not fixable overnight or even in the next few years.”

Tactical Shift

The Paris attacks, right after the downing of a jet of Russian tourists over the Sinai and suicide bombings in Lebanon and Iraq, indicate a sharp tactical shift for Islamic State. It had mostly limited its brutality to building a state-like Islamic caliphate in parts of Syria and Iraq, rather than sending its battle abroad, like its rival al Qaeda.

That change increases the challenge to the U.S.-led coalition, especially with Obama having staked his foreign policy legacy on getting the U.S. out of wars, not entering new ones. Current and former U.S. officials say coalition governments now face the threat of more attacks in their cities. Many also argue that the U.S. and its partners have little choice but to increase their military commitment.

“Unless you leave planet Earth, you can’t avoid this,” said Michael Chertoff, who has a security consulting firm and was Secretary of Homeland Security under George W. Bush.

A day before the Paris assault, Obama told ABC News that Islamic State had been “contained” in Iraq and Syria. Indicating how the Paris attacks are affecting the discussion, Hillary Clinton, his former secretary of state and the frontrunner for the Democratic presidential nomination, disagreed at a debate on Saturday. Islamic State “cannot be contained,” she said. “It must be defeated.”

Some Successes

The coalition had hoped to do that with more than 8,000 air strikes at a cost of $5 billion, according to the Pentagon. And there have been successes. The group lost the northern Iraqi city of Sinjar 48 hours after troops loyal to Syrian President Bashar al-Assad, backed by Russian air strikes, broke a two-year siege on the Kweiris military base in Aleppo province.

In fact, some analysts see the Paris attacks as a sign of IS despair after those defeats. “Islamic State’s decision to push the button is related to the pressure on it,” said Sami Nader, head of the Beirut-based Levant Institute for Strategic Affairs.

But given the nature of asymmetrical warfare, it would be hard to argue that the fight against IS has been successful. Recently, the U.S. abandoned a train-and-equip program for moderate rebels and sent 50 Special Forces troops into Syria to assist with strikes. And those who follow the fight against terrorists say the Paris attacks showed sophistication. “This was no small plot,” said Patrick Skinner, a former CIA officer who directs special projects at the Soufan Group, a security consulting firm. “To be able to pull this off in a modern security state like France – which has really great intel and great security – it’s just worrisome.”

Combat Operations

The answer, said Thomas Donnelly of the conservative American Enterprise Institute, is “large-scale combat operations” in Iraq and Syria. It would take “more years of heavy combat than we’ve seen before” and “decades,” to properly re-integrate alienated Sunni populations that have sometimes backed IS. The initial stage would cost more than $1 trillion over several years, he estimates, and 150,000 troops.

“Anything less than military engagement is likely to be useless,” Donnelly said. “It’s a war.”

The problem is that, even with a massive troop commitment, such a war would only prove successful if led by regional powers, none of which are willing. In fact, the paradox is that IS has taken root in the region precisely because of the vacuum created by other disputes — intra-Syrian, Sunni-Shia, Turkish- Kurd, Iranian-Saudi.

“For the Saudis, countering Iran is more important” than fighting Islamic State as evidenced by the war in Yemen, said Kamran Bokhari, a lecturer of national security at the University of Ottawa. Egypt is facing a balance of payments crisis and is reeling from its own terror attacks, while Iraq is struggling to cope with a slump in oil prices and its own war. And Turkey, which might be best-placed to assist, prefers to attack Kurdish forces in Iraq and in its southeast.

Nobody in the region is really fighting Islamic State, Bokhari said.

Other Steps

Some say steps short of full military intervention would help. Riedel of the Brookings Institution, urges the killing of the group’s leader, Abu Bakr al Baghdadi. Michael O’Hanlon, a security analyst at Brookings, suggests giving up the vision of a united Syria. That would allow the coalition to work with the Kurds in certain areas, create no-fly zones and bolster moderate groups. NATO and allied forces could be sent in “to catalyze training and ensure humanitarian relief,” while the U.S. could send Iraq more trainers and Special Forces to conduct raids. O’Hanlon estimated this would require fewer than 10,000 troops in each country.

Riad Kahwaji, head of the Institute for Near East and Gulf Military Analysis in Dubai, said it’s crucial that any ground troops be Sunni. Right now, it’s largely Shiites, including Iran and Hezbollah, fighting the militants in Iraq and Syria. That plays into Islamic State’s narrative that it’s defending Sunnis against a Shiite onslaught, he said.

If there isn’t a change in strategy, Kahwaji said “the war in Iraq and Syria is going to be in many places in Europe and will even spread to the U.S.”

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